There is good evidence that transformed cells exhibit increased levels of plasminogen activator activity compared to normal cells. The major difficulty in proving a correlation between increased levels of plasminogen activator activity and tumorigenicity is there is no single assay for the plasminogen activator of cells which is both sensitive and quantitative. We have synthesized a fluorogenic substrate for plasmin which is 20-fold more sensitive than ones currently available and have used it to develop an assay for the plasminogen activator activity of transformed cells. The assay is cheap, easy to perform, quick, sensitive and highly quantitative. The macroscopic constants Km and Vmax can be measured for plasminogen and the plasminogen activator of cells without destroying the cells. The assay is sensitive enough to yield a fluorescent signal from as few as 250 cells. A variation of the assay, with a fluorescence microscope, should allow the quantitative measurement of the plasminogen activator activity of single cells. The assay will be used to determine the degree of correlation between plasminogen activator activity and tumorigenicity. It will also be used to study the factors that influence the magnitude of plasminogen activator activity in different cell lines, as well as the regulation of that activity within a cell line and to screen for viral temperature-sensitive transformation mutants. A major objective is to develop a carcinogen assay that will be as simple to perform as is the Ames' assay for mutagens.